One further question.
When she buried herself in books, someone came in and stole her purse.
Is it also necessary to backshift this sentence？Does "When she buried herself in books" suggest habit in the past？
Hi, Robby zhu,
I hope you (and David) don't mind my stepping in.
"When she buried herself in books" does suggest a habit in the past. The zero article before "books" makes the habitual meaning even stronger. But then you have a main clause that seems highly unlikely to have occurred repeatedly, unless we add "always":
- When she buried herself in books, someone always came in and stole her purse.
If you want to refer to a one-time occurrence, then you need the past perfect, or else to change "when" to "after":
- When she had buried herself in her books, someone came in and stole her purse.
- After she buried herself in her books, someone came in and stole her purse.
Notice I've changed the generic "books" to "her books." I think "books" would be possible if there were some previous context referring to some other reading material, and in this case I believe the past simple would be fine (do you agree, David?):
- She had been thumbing through some magazines at ease for hours but when she buried herself in books someone came in and stole her purse.
Unlike "because," which introduces a durative action as a result of which she never became aware of the rain, "when" can introduce a short action to mean that the moment she concentrated her attention on books, someone seized her distraction to take her purse.